Process for making green briquettes for forming Si or SiC

ABSTRACT

A process and apparatus for the preparation of green briquettes for the production of silicon or silicon carbide or ferrosilicon in electric pit furnaces, quartz sand, a carbon carrier and a bituminous binder being mixed together, the mixture being shaped into blanks from which the green briquettes are formed by heat treatment. The process is performed with the use of blanks which are free from melted bituminous caking coal and whose specific weight is made greater than the bulk density of quartz sand by adjustment of the proportions of the mixture and by compacting. The blanks are introduced for the heat treatment into a heated rotating drum furnace whose lower part is filled with quartz sand to an extent such that the heat treatment proceeds in a dip bed of quartz sand.

CROSS REFERENCE TO RELATED SUBJECT MATTER

This application is related to Ser. No. 865,164 filed 20 May 1986 (nowU.S. Pat. No. 4,820,341) and 275,159 filed 26 Nov. 1984 now U.S. Pat.No. 4,364,974, and related to U.S. Pat. No. 4,366,137 issued 28 Dec.1982 and 4,364,974 issued on 21 Dec. 1982.

FIELD OF THE INVENTION

My present invention relates to a process and apparatus for theproduction of green briquettes for the formation of silicon or siliconcarbide or ferrosilicon in low-shaft electric furnaces.

BACKGROUND OF THE INVENTION

In the production of silicon, silicon carbide or ferrosilicon, quartzsand, a carbon carrier and a bituminous binder can be mixed together andthe mixture shaped into blanks from which green briquettes are formed.

As a rule, quartz sand in a range of grain sizes of from 0.05 to 0.2 mmis used for the preparation of briquettes of this kind, a variety ofparticle size ranges employed between these limits. Of course, for thepreparation of ferrosilicon finely divided iron or finely divided ironoxide can also be added to the green briquettes.

Green briquettes for the purposes specified must comply with specialchemical and physical requirements. For example, and as can be gatheredfrom U.K. Patent No. 2 084 122, an important chemical consideration inthe preparation of silicon is that as the green briquettes pass throughthe low-shaft electric furnace, silicon carbide may first form in thetop part in accordance with the equation:

    SiO.sub.2 +3C=SiC+2CO

The total carbon content of the individual green briquettes must beadjusted correspondingly It is usually adjusted to a hyperstoichiometriclevel (cf. U.K. Patent No. 2 150 128). From the physical angle it mustbe ensured that, as the green briquettes pass through the furnace, theydo not disintegrate, especially during the reduction to SiC, in order toreact in the bottom part of the furnace with molten quartz of the chargein accordance with the equation:

    SiO.sub.2 +SiC=3Si+2CO 2CO

Of course, other reactions also occur in the furnace. One of them is theformation in accordance with the formula:

    SiO.sub.2 +C=SiO+CO

at the high furnace temperatures of silicon monoxide, some of which islost, with detriment to the yield and the heat balance. In practice, theonly possible way of preparing green briquettes for the production ofsilicon in a manner satisfying chemical and physical requirements hasbeen to use hot briquetting (c.f. U.K. Patent No. 2 088 840).

This hot briquetting process produces, at temperatures around 500° C.,briquettes containing molten or fused caking coal as the bituminousbinder. The melting point of this coal binder, which may be between 350°and 450° C., must be exceeded during briquetting if the molten coal isto be distributed satisfactorily in the material to be bound and if theblank is to be consolidated by the formation of semicoke.

A very important factor in briquette quality has been a special heattreatment in which the blanks are stored substantially at theirbriquetting temperature in insulated bunkers for several hours--i.e., aheat treatment or tempering and using the intrinsic heat of the blanksthereby doubling the strength of the briquettes.

However, it has been found that preparing green briquettes for theproduction of silicon or silicon carbide or ferrosilicon by hotbriquetting is relatively costly.

Another disadvantage when there are stringent purity requirements forthe products to be produced from the briquettes is that impurities maybe taken up into the briquettes during hot briquetting either from thecaking coal or from metal abrasion of the apparatus in which the hotbriquetting is carried out. Similar considerations apply to theproduction of silicon carbide or ferrosilicon.

The term "ferrosilicon" comprises for the purposes of the invention ametal alloy containing silicon, more particularly, with iron as metal.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an improved process formaking green briquettes which are completely satisfactory from thechemical or physical viewpoints are prepared without the use of hotbriquetting.

Another object of my invention is to provide an improved apparatus forcarrying out the method.

SUMMARY OF THE INVENTION

According to the invention, therefore, the process is performed bymaking shaped bodies which are free from melted or fused bituminouscaking coal and whose specific gravity is made greater than the bulkdensity of quartz sand by adjustment of the proportions of the mixtureand by compacting.

The shaped bodies are introduced for the heat treatment into a heatedrotary-kiln furnace whose lower part is filled with quartz sand to anextent such that the heat treatment proceeds in a dip bed of quartzsand. The term "free from melted or fused bituminous caking coal" meansthat caking coal is virtually absent and, if present at all, is notpresent in a quantity such that the other carbon carriers and the quartzsand are bound by way of caking coal.

The invention is based on my discovery that shaped bodies of relativelylow green strength which have been molded without caking coal--i.e.without being hot briquetted by means of a bituminous binder--can be sohardened in a rotary kiln furnace without decomposing that the finishedbriquettes meet all physical and chemical requirements when the heattreatment is given in a quartz sand dip bed and, therefore, virtuallywith the exclusion of air.

The grain size of the dip bed can correspond to the grain size of thesand mentioned previously.

The treatment time of the blank in the dip bed is adjusted to ensureadequate hardening. The hardening time is, for example, in the range ofsome 45 to 60 minutes. This is readily possible with modern rotary kilnfurnace technology. The shaped bodies and, therefore, the finishedbriquettes can have diameters varying from a few millimeters to a fewcentimeters. They can be shaped in a pelleting plant as round pellets orpressed to other shapes by the shaping tools of a press, for example, tothe shape of ovoid briquettes.

Advantageously, the process makes use of shaped bodies containing from35 to 65 weight % of quartz sand and having a specific gravity of from1.4 to 1.6 g/cm³, while the bulk density of the quartz sand can ofcourse be approximately 1.2 g/cm³ or a little higher.

The temperature in the furnace should not be too high. Preferably, therevolving tubular furnace and therefore the quartz sand dip bed areheated to a final temperature of up to 500° C. Optimally, in the heattreatment the blanks undergo adequate degasification of heavyhydrocarbons and adequate low-temperature carbonization of thebituminous binder and carbon carrier in them.

Advantageously, the furnace is filled with quartz sand to an extentsufficient to ensure that the blanks do not rub against one another inthe dip bed. According to the invention, to achieve this the furnace isfilled with quartz sand at least to such an extent that the volumethereof is more than twice the void volume in a bulk fill of thecorresponding quantity of shaped bodies in the furnace. The void volumeis the sum of the volumes of the voids between the shaped bodies in abulk fill.

The dip bed can according to the invention remain in the furnace, inwhich event just the finished green briquettes are removed by suitablemeans. Since the briquettes thus removed always carry away some of thesand from the dip bed, corresponding topping-up with fresh quartz sandis necessary.

In a preferred embodiment of the invention, which is distinguished bythe simplicity of the process and which leads to further advantages tobe discussed hereinafter, a considerable quantity of quartz sand of thedip bed is discharged from the furnace together with the finished greenbriquettes, at least some of such sand being returned hot to thefurnace, the remainder being replaced by fresh quartz sand.

In this case, it is possible for the dip bed to have a specialtemperature profile, preferably so that the mixing of the recirculatedhot quartz sand with the fresh quartz sand at the furnace entry producesin the blank-heating dip bed a temperature of approximately 200° C.,which the heat of the furnace increases to approximately 500° C. at thefurnace exit.

During the heat treatment first the heavy and then the lighterhydrocarbons evaporate from the shaped bodies. These hydrocarbons crackin the dip bed. Consequently, the hot quartz sand can be recirculated asoften as is necessary for a strong envelope of carbon, in the form ofsolid crack carbon, to be formed on the discrete grains of quartz sand.

The simplest way to achieve this result is for the discrete sand grainsto be recirculated about three times on the statistical average.

The grains having such an envelope can be used in a special way. To thisend, according to the invention, the process is performed using blankswhose quartz sand content consists at least to some extent ofrecirculated quartz sand with a carbon envelope. Of course, theremaining cracking products are burned above the dip bed.

Advantageously, to produce green briquettes substantially devoid ofimpurities derived from the carbon carriers or the bituminous binders,the process is performed using shaped bodies having oil coke as carboncarriers and/or petroleum-based pitches as bituminous binders.

Petroleum-based pitches of use in the process according to the inventionare commercially available with ash contents below 0.01 weight %, sothat they contribute virtually no impurities. The coke yield of suchpitches may reach 70T.

Petroleum coke, which is particularly suitable for the process accordingto the invention, has less than 0.2 weight % of undesirable constituentsSimilar considerations apply to quartz sand. These slight impuritieshave negligible effects. The petroleum coke is of course ground down tosubstantially the same grain size as the quartz sand. Preferably, thegrain size is below 1 mm.

As previously stated, in the process according to the invention theblank can be shaped in various ways, for example, by means of apelleting device outputting spherical pellets. However, it is simpler towork with blanks shaped by means of a briquetting press. In this eventthe specific weight of the blanks can be controlled in a simple way bythe pressing step.

The invention also relates to an apparatus for performing the processdescribed and comprising means for shaping the quartz sand blanks, acarbon carrier and a bituminous binder and means for heat-treating theblanks.

In such apparatus, the means for heat-treating the blanks can be arotary-drum furnace having an entry for the blanks and an exit throughwhich the green briquettes leave, the furnace being adapted to be heatedto a sand bed temperature of up to approximately 500° C. by burnerswhich burn into the furnace chamber, for example, for solid fossil fuelsor gas fuels, and by combustion of the volatile constituents withadditional air. The furnace has a quartz sand dip bed for the blanks,such bed partly filling the oven chamber in the region below the burners

Preferably the furnace has a recirculating facility for the quartz sandof the dip bed, such facility interconnecting the furnace entry andfurnace exit, the sand being adapted to be discharged together with thefinished green briquettes and to be removable therefrom by screening andto be reintroducible into the furnace through the entry, and means formixing-in fresh quartz sand. Of course, the gases and vapors evolved inthe heat treatment and elsewhere in the apparatus are intercepted andcleaned or made harmless The invention also relates to green briquettesfor the production of silicon or silicon carbide or ferrosiliconincluding silicon metal, as prepared by the process hereinbeforedescribed.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of my inventionwill become more readily apparent from the following description,reference being made to the accompanying highly diagrammatic drawing inwhich:

The sole FIGURE of which is a diagram which shows an apparatus forperforming the process according to the invention.

SPECIFIC DESCRIPTION AND EXAMPLE

The apparatus illustrated is of use for the preparation of greenbriquettes for the production of silicon or silicon carbide.

Basically, the apparatus comprises means 1 for molding blanks consistingof quartz sand, a carbon carrier and a bituminous binder, and means 2for heat treating the blanks

The means 2 take the form of a rotary tube furnace 3 having an entry 4for the blanks and an exit 5 through which the green briquettes leaveafter a programmable treatment time.

Burners 6 which burn into the furnace chamber heat the furnace 3 to atemperature of up to approximately 500° C. The furnace 3 has a quartzsand dip bed 7 for the blanks, the bed 7 partly filling up the furnacechamber in the region below the burners 6.

The furnace 3 also has a recirculating facility 8 for the quartz sand ofthe dip bed 7, such facility interconnecting the furnace entry 4 andfurnace exit 5, the sand being adapted to be discharged together withthe finished green briquettes and to be removable therefrom by screeningby the screening facility 9.

The quartz sand removed by screening can be reintroduced hot through theentry 4 into the furnace 3.

Also disposed at the furnace entrance 4 are means 10 for introducing andmixing-in fresh quartz sand.

The means 1 for preparing the blanks are disposed upstream of theapparatus hereinbefore described, are combined therewith and comprise inthe embodiment shown, two rotary drum mixers 11, a cooler 12 and a press13 for shaping the blanks Petroleum coke is used in the embodiment.

The petroleum coke enters the first mixer 11 which is lined, forexample, with SiC plates Since petroleum coke contains between 6 and 10%of water, it is dried and heated in the first mixer. Heating and dryingare effected by using a proportion of the quartz sand used as heatvehicle in the furnace 3. The sand thus used has a temperature of 500°C.

The evaporation of the water and the heating requires so much energythat for an approximately half-and-half mixture of the petroleum cokeand quartz sand an end temperature of approximately 150° C results. Ifthis temperature is too low for the next step in which the binder mustbe admixed and intensively distributed, waste gas can be conveyed fromthe furnace 3 through the first mixer 11 in which it yields heat and issaturated with vapors. The waste gas has a temperature of approximately650° C.

Also, thanks to its reduced oxygen content of less than 1.5%, the wastegas can render the atmosphere in the first mixer 11 inert(monooxidizing), thus minimizing dangerous situations which might becaused by powdered petroleum coke. After the mixer 11 the waste gasgoes, for example, by way of a loose-fill filter having an independentsystem of filling of petroleum coke or quartz sand, to the flue.

The mixture of sand and petroleum coke, which is now at a temperature ofapproximately 200° C., goes to the second mixer 11 in which the binderis sprayed at a temperature of approximately from 180° to 200° C. Thebinder may be a petroleum-based pitch. The temperatures are such thatthe viscosity is low enough to prevent caking and to ensure finedistribution in the mixture of solids. The second mixer 11 also has alining of SiC plates. This lining feature is used wherever possible inorder to reduce wear to a minimum and to enable any material which doesrub off to be incorporated readily in the product.

The mixture issuing from the second mixer 11 is too hot for briquettingsince it sticks readily to the press surface. The mixture is thereforecooled in the cooler by from 30° to 50° C., depending upon the nature ofthe binder. The cooler 12 also has SiC plates. The cooling can be bycountercurrent air cooling. The quantity of cooling air depends upon therequired end temperature of the mixture to be briquetted.

The mixture to be briquetted goes to the press 13. The same is enclosedand has controlled ventilation. The press 13 outputs the blanks whichare hardened in the heat treatment means 2. Hardening in the presentcase denotes degassing and pre-coking of the binder and petroleum cokein the briquette.

The furnace 3 is less than half-full of quartz sand used in thepreparation of the green briquettes. The furnace 3 has, for example, alength of from 20 to 30 m and an internal diameter of from 2 to 3 m. Thelength depends upon the temperature profile for heating the blanks. Thefurnace contents are heated by a direct flame which is maintained in theremaining free space.

The first source of fuel for the flames is by way of the burners 6 whichflame into the oven 3 and the second source is the volatile constituentsof the blanks When the blanks are heated the binder starts to evolveheavy hydrocarbons at temperatures above 250° C. This evolutioncontinues substantially up to 450° C., whereafter evolution shifts tosmaller molecules such as methane and hydrogen. The sum of the fuels ismatched in dependence upon the initial and final temperature of thesolids and upon the total throughput.

The quartz sand filling in the embodiment comprises two components--aproportion of hot recycled quartz sand, and a proportion of fresh sand.The fresh sand is directly proportional to output--i.e., the higher thehourly output of green briquettes, the more fresh sand is requiredConversely, an increase in throughput leads to an increase in theevolution of volatile constituents which burn. Throughput is thereforelimited by the permissible combustion chamber loading of the furnace 3.On the assumption that an end temperature of the solids of approximately500° C. is required, the temperature of the hot gases must be at least800° C., preferably in concurrent operation.

The blanks can be given a defined treatment in the dip bed 7. Hence thespecific weight of the blanks can be selected. This specific weight canbe adjusted between 1.35 to 1.55 g/cm³ in dependence upon the proportionof quartz sand and upon the briquetting pressure in the press 13. Thiscompares with a bulk density of moving quartz sand with a narrow band ofgrain sizes of from 1.2 to 1.4 g/cm³.

This comparison shows that the blanks can be degassed very thoroughly inthe quartz sand of the dip bed. The blanks are therefore protectedagainst burning away.

The strength reached is surprising. Of equal importance is a secondaryeffect. After heavy hydrocarbons have been evolved from the blanks,these organic substances are compelled to escape through the hot quartzsand of the bed 7 into the free space of the furnace 3. In contact withthe quartz sand, these substances are to some extent so decomposed thatthe quartz sand absorbs carbon and lighter hydrocarbons and hydrogen canleave the quartz sand and burn thereabove. This coating of the quartzsand with crack carbon leads to a quartz sand which ceases to offer thehardness of quartz to pressing operations in the press 13 since thesurface of the sand grains has a sufficiently thick layer of relativelysoft carbon.

Loading in the press 13 lasts only for fractions of a second and doesnot lead to contacting between steel and quartz which would otherwiseerode the press 13.

The bonding of the coated quartz sand to the finished green briquette isthe second advantage associated with reduction in a low-shaft electricfurnace. Previously briquettes prepared with binders were unsatisfactoryin low-shaft electric furnaces first because they softened with heating,second because they disintegrated due to the binder filling and thirdbecause the SiC reaction destroyed the binding residual structures. Theinvention obviates all these disadvantages. The hardening describedobviates softening and distillation of the bituminous binder in thelow-shaft electric furnace. The SiC reaction which in the event ofdirect contact between quartz sand and coke from the binder destroys thestabilizing hydrogen structure of the briquette above 1500° C. proceedsinitially with the carbon enveloping the grain of quartz sand. This doesnot impair the structure of the coke which has evolved from the binder.Briquette strength stays constant over a longer time or up to a highertemperature than previously in the low-shaft electric furnace, at leastup to the melting point of the quartz sand in the briquette, which isabove 1700° C.

After the quartz sand and the finished green briquettes have passedthrough the furnace 3, they are separated from one another by screening.The green briquettes go to a bunker with protection from oxygen. Thatproportion of the quartz sand which is required for production issupplied to the first mixer 11 and mixed with petroleum coke, the samebeing dried in this step. The mixture proportions can be chosen freely.It has been found in practice, however, that for silicon productionbriquette compositions representing a molar ratio of quartz sand to C offrom 1 to 5 or 6 mol are advantageous. For silicon carbon production aratio of quartz sand to be fixed carbon of 1:3 can be used. For otherproductions which have not been tested proportions can be varied to suitrequirements. The limit is the specific weight of the blanks whichensures that the blanks float while dipped in the dip bed 7 right fromthe start.

I claim:
 1. A process for making green briquettes for use in theproduction of silicon, silicon carbide, or ferrosilicon in a low-shaftelectric furnace, the process comprising the steps of:(a) preparing amixture substantially free from molten bituminous caking coal butcontaining quartz sand, a carbon carrier and a bituminous binder; (b)shaping said mixture into blanks with compaction and controlling thecomposition of said mixture and the degree of compaction of said blanksso that said blanks have a specific weight less than the bulk density ofquartz sand; and (c) hardening said blanks to form said green briquettesby heat treating said briquettes in immersion into an agitated sand bathby introducing said blanks into a heated rotating-drum furnace having alower part filled with quartz sand so that said blanks are treated in adip bed of the quartz sand for a period of time and at a temperaturesufficient to harden said blanks.
 2. The process defined in claim 1wherein said blanks contain from 35 to 65 weight % of quartz sand andhave a specific weight of from 1.4 to 1.6 g/cm³.
 3. The process definedin claim 1 wherein the furnace and therefore the quartz sand dip bed areheated to a final temperature of up to 500° C.
 4. The process defined inclaim 1 wherein the heat treatment is carried out so that the blanksundergo very complete degasification of heavy hydrocarbons andlow-temperature carbonization of the bituminous binder and carboncarrier in them.
 5. The process defined in claim 1 wherein the furnaceis filled with quartz sand to such an extent that the volume thereof ismore than twice the void volume in a bulk fill of the blanks.
 6. Theprocess defined in claim 1 wherein the quartz sand of the dip bed isdischarged from the furnace together with the finished green briquettes,at least some of such sand being returned hot to the furnace, theremainder being replaced by fresh quartz sand.
 7. The process defined inclaim 6 wherein the mixing of the recirculated hot quartz sand with thefresh quartz sand at the furnace entry produces in the dip bed atemperature of approximately 200° C., which is increased up toapproximately 500° C. at the furnace exit.
 8. The process defined inclaim 6 wherein the hot quartz sand is recirculated as often as isnecessary for a strong envelope of carbon to be formed on discretegrains of quartz sand.
 9. The process defined in claim 8 wherein thediscrete sand grains are on the statistical average circulated aboutthree times.
 10. The process defined in claim 8 wherein said blankscontain quartz sand consisting at least to some extent of recirculatedquartz sand with a carbon envelope.
 11. The process defined in claim 1wherein oil coke is said carbon carrier.
 12. The process defined inclaim 1 wherein petroleum-based pitch is said bituminous binder.